Injection-stretch blow molding (ISBM) is a process widely used for the production of containers, such as bottles, using thermoplastic polymers. The process includes the steps of preparing a pre-form by injection molding and then expanding the pre-form to the desired final shape. In general, one distinguishes one-stage and two-stage processes. In the one-stage process the steps of producing the pre-form and expanding the pre-form to the desired final shape are performed in the same machine. In the two-stage process these two steps are performed in different machines, in some cases even in different geographical locations; the pre-form is allowed to cool to ambient temperature and is then transported to a second machine where it is reheated and expanded to the desired final shape. Due to reasons of production speed and flexibility the two-stage process is preferred for larger production volumes.
Recent progress in development has made polypropylene a viable alternative to polyethylene terephthalate (PET) for injection-stretch blow molding (ISBM). Due to their good optical properties propylene-ethylene random copolymers are the preferred polypropylene grades.
For the injection molding of polypropylene it is well known to improve the impact performance, while also having good optical properties, by the addition of a polyethylene, which has been produced using a metallocene catalyst.
For example, EP-A-151741 to Mitsui discloses single-stage manufacturing of articles by ISBM. These articles are prepared from propylene-ethylene random copolymers having a melt flow index of from 4 to 50 g/10 min and containing a nucleating agent. WO95/11791 to Bekum is directed to a two-stage process for preparing articles by ISBM. The preferred resin is an ethylene-propylene copolymer containing more than 50 wt % of propylene and having a melt index of from 10 to 20 g/10 min.
WO 2005/005143 to Total Petrochemicals discloses blow-molded containers made from a blend of polypropylene and a metallocene polyethylene to improve the impact strength.
The polypropylenes presently used in injection-stretch blow molding applications allow for the production of containers with good optical properties at industrially viable production rates. However, as compared to other polymers used in injection-stretch blow molding, polypropylene suffers from a lack of the combination of high rigidity and high ESCR, as well as high impact strength, particularly at lower temperatures.
Thus, there is an interest for improving the impact performance, rigidity and ESCR of injection-stretch blow molded containers. A balance has to be found between the high fluidity required for the first step to form the preform and the lower fluidity required for the second step when blowing the preform.
JP2000086722 to Asahi discloses the use of high-density polyethylene, preferably prepared with a metallocene catalyst, suitable for injection stretch blow molding.
JP2000086833 to Asahi discloses the use of resin compositions suitable for injection stretch blow molding at a high stretch ratio, comprising a polyethylene prepared with a metallocene catalyst and a polyethylene prepared with a chromium catalyst.
JP9194534 to Mitsui discloses the use of a polyethylene-based resin for injection stretch blow molding having a density of 0.940 to 0.968 g/cm3 and a melt flow index of 0.3 to 10 g/10 min (ASTM D1238 at 190° C. and 2.16 kg).
It is an aim of the invention to provide a polyethylene resin for injection stretch blow moulding with a broad processing window.
It is also an aim of the invention to provide a polyethylene resin for injection stretch blow moulding with good process stability.
It is an aim of the invention to provide a polyethylene resin for injection stretch blow moulding with a high environmental stress crack resistance (ESCR measured with 100% Igepal CO-630). The environmental stress crack resistance is advantageously of at least 100 h, preferably at least 400 h.
In addition is an aim of the invention to provide a polyethylene resin for injection stretch blow moulding with a high impact resistance.
Furthermore, it is an aim of the invention to provide a polyethylene resin for injection stretch blow moulding with high rigidity.
In addition, it is also an aim of the invention to provide a polyethylene resin for injection stretch blow moulding to prepare containers with a high top load. The top load is the ability of a standing bottle to withstand the weight of other bottles on pallets.
It is further an aim of the invention to provide a polyethylene resin for injection stretch blow moulding to prepare containers with good thickness repartition.
It is additionally an aim of the invention to provide a polyethylene resin for injection stretch blow moulding to prepare containers with good surface aspects.
It is furthermore an aim of the invention to provide a polyethylene resin for injection stretch blow moulding to prepare containers with good finishing for molded drawings.
Finally, it is also an aim of the invention to provide a polyethylene resin suitable for injection stretch blow moulded containers for consumer packaging, in particular for cosmetics and detergents.
At least one of these aims is fulfilled by the resin of the present invention.